Granular Landslides Research Articles

The energy transfer from granular landslides to water bodies explained by a data-driven, physics-based numerical model

Landslides falling into water can trigger tsunamis, which are particularly destructive in the proximity of the landslide impact and in narrow water bodies. The energy transfer mechanism between landslide and water wave is complex, but its understanding is of fundamental importance for the numerical modeling which aims to predict the induced wave hazard. In order to study the involved physical processes, we set up an experimental facility consisting of a landslide generator releasing gravel at high speed in a wave basin. With the aim of estimating the landslide–wave energy transfer, we implemented a simplified 1D conceptual model of landslide motion, including the 3D landslide deformations. We optimized the model with the experimental results. The model results explain that the deformable landslide has an average drag coefficient of 1.26 and a relatively inefficient energy transfer from landslide to wave. Of the landslide energy at impact, the 52% is dissipated by Coulomb basal friction between the slide and the water basin bottom, 42% is dissipated by other processes, including turbulence, and only the remaining 6% is transferred to the wave thus formed.

Multi-phase flow simulation of impulsive waves generated by a sub-aerial granular landslide on an erodible slope

This study uses a multi-phase flow model to examine the impulsive waves generated by collapse of steep slopes at a water body. Possible effects of slope’s erodibility on the motion of the landslide and the resulting waves are investigated by comparing a sub-aerial landslide on a rigid slope to that on an erodible slope. Key parameters that are relevant to the generation of the impulsive waves are examined, including the time series of the elevation and location of first wave crest, the location and thickness of granular flow front, submerged granular volume, and potential and kinetic energy for water and granular landslide. Before it is free from the influence of the granular front, the generated wave is a forced wave traveling at the speed of the granular flow front. The first wave crest is about to leave the granular flow front when the speed of the granular front is close to the celerity at which a free long wave can propagate in the water above the granular front, and the maximum height of the first wave crest is reached at the time when the first wave has completed its transition from a forced wave to a free wave. It appears that the erodible slope can increase (1) the volume of the granular material plunged into water, (2) the thickness of granular flow front, and (3) the height of the first wave generated by the landslide. However, the total volume of the granular material plunged into water is not directly related to the height of the first wave.

Mobility of dry granular flows of varying collisional activity quantified by smart rock sensors

Highly instrumented particles (i.e., “smart rocks”) were included in monodisperse dry granular landslide experiments to quantify the collisional nature of such flows and to investigate the influence of collisional flow on the mobility of landslides. The total number of particles comprising a constant source volume of 0.4 m3 was varied by filling the volume with monodisperse particles of nominal diameters of 3, 6, 13 or 25 mm. Successively raising the total particle count resulted in flows that were increasingly thick relative to the respective particle size. Raw resultant acceleration data from the embedded smart rock sensors indicate that for each increase in grain size, there were increases in both the magnitude and frequency of particle collisions. Light detection and ranging (LiDAR)-generated point clouds of the landslide deposits indicated that increases in mobility and spreading, compared using differences in travel angle, were directly proportional to increases in collisional activity. By changing the size of the landslide particles from 3 to 25 mm, the travel angle at the gravity centre (αg) was observed to decrease from 27.8° to 25.3° (Δαg = −9.0%) and the Fahrböschung angle (α) was observed to decrease from 25.0° to 21.4° (Δα = −14.4%).

Barrier lake formation due to landslide impacting a river: A numerical study using a double layer-averaged two-phase flow model

A granular landslide impacting a river may lead to the formation of a landslide dam blocking the streamflow, and subsequently create a barrier lake. Should a barrier lake outburst, the flood may be destructive and spell disastrous consequences downstream. The last decade or so has witnessed a number of experimental and numerical investigations on barrier lake outburst flooding, whilst studies on barrier lake formation remain rare – a physically enhanced and practically viable mathematical model is still missing. Generally, barrier lake formation is characterized by multi-physical, interactive processes between water flow, multi-sized sediment transport and morphological evolution. Here, a new double layer-averaged two-phase flow model is proposed, which is an advance on existing continuum models that involve a single-phase flow assumption and presume a single sediment size, and discrete models that preclude fine grains and assume narrow grain size distributions. The proposed model is first validated against data from previous laboratory experiments of waves due to landslides impacting reservoirs and landslide dam formation over dry valleys. Then it is applied to explore the complicated mechanism and threshold for barrier lake formation. The water and grain velocities are shown to be disparate, characterizing the primary role of grains in driving water movement during subaqueous landslide motion and also demonstrating the need for a two-phase flow approach. The grain size effects are revealed, i.e., coarse grains and grain-size uniformity favour barrier lake formation. A new threshold condition is proposed for barrier lake formation, integrating the landslide-to-river momentum ratio and grain size effects. The present work facilitates a promising modelling framework for solving barrier lake formation, thereby underpinning the assessment of flood hazards due to barrier lakes.

Global optimization for data assimilation in landslide tsunami models

The goal of this article is to make automatic data assimilation for a landslide tsunami model, given by the coupling between a non-hydrostatic multi-layer shallow-water and a Savage-Hutter granular landslide model for submarine avalanches. The coupled model is discretized using a positivity preserving second-order path-conservative finite volume scheme. Then, the data assimilation problem is posed in a global optimization framework. Later, multi-path parallel metaheuristic stochastic global optimization algorithms are developed. More precisely, a multi-path Simulated Annealing algorithm is compared with a multi-path hybrid global optimization algorithm based on coupling Simulated Annealing with gradient local searchers.

Multi-phase-flow modeling of underwater landslides on an inclined plane and consequently generated waves

An underwater landslide is a complex multiphase flow phenomenon with potential to cause large waves and destruction. Particle size is an important parameter that affects flow behaviors of multiphase flows. This numerical study examines underwater granular landslides on an inclined plane and their resultant waves with the purpose of exploring how the particle size affects landslide kinematics and wave amplitudes. A rheology-based multi-phase flow model is employed, newly incorporating the Navier-slip condition. Smooth and rough beds that reflect ideal and practical conditions, respectively, are simulated. Five types of particle are used; they are very fine sand, medium sand, coarse sand, granules, and pebbles. The granular masses are initially in the loose-packing condition. Our numerical results demonstrate that a smooth bed results in the faster landslides and the larger impulsive waves than a rough bed. The effects of particle size differ between the two bed conditions. For the rough bed, the relationship between the landslide speed and the particle size is negative, and so is the relationship between the wave amplitude and the particle size. For the smooth bed, the two relationships become concave and non-monotonic.

Experimental analysis of tsunamis generated by the impact of landslides with high mobility

Landslides into water can generate massive tsunamis which are major natural hazards in coastal regions. In this study, waves were generated in a series of 41 laboratory experiments by releasing 4 different slide volumes ranging from 0.1-0.4 m3 of highly mobile material down a 6.73 m long slope into a reservoir of depth varying from 0.15-0.65 m, to achieve a wide range of dimensionless landslide parameter values. Water is used as the sliding material and with zero internal shear strength it is representative of the upper limit of high landslide mobility. The slide characteristics of thickness and velocity were measured at impact using high speed cameras and the time series of the resulting changes in water surface elevation were measured using nine wave probes along the 33.8 m long flume. The experimental results indicate that in the near-field the maximum wave amplitude is dependent on the landslide thickness and velocity and is relatively independent of the reservoir water depth. As waves propagate to the far-field, the depth-limited breaking reduces the wave amplitude, such that the maximum wave amplitude is highly dependent on the reservoir depth. The wave breaking limit, which differentiates the breaking from the non-breaking waves, is defined by the relationship am/h = 0.6 that is very closely adhered to for all source volumes and reservoir depths. The time and length over which the waves are generated are determined from the digital imagery, and are used to describe the scales of the momentum transfer. These observations are compared with previously published empirical and theoretical equations for granular landslides and positively buoyant avalanches, indicating that water is a useful source material for simulating highly mobile landslides. Since these parameters are difficult to obtain in field cases, a simplification of the theoretical equation is presented and this yields reasonable results for the maximum wave amplitude generated by slides with high mobility.

Multi-phase flow modeling of submarine landslides: Transformation from hyperconcentrated flows into turbidity currents

Sedimentary density flows are major agents of sediment transport on oceans. Occurring after submarine landslides, the sedimentary density flows experience a series of dilution processes. Such flows, then, transform from the hyperconcentrated flows into the concentrated ones, and finally turn into the turbidity currents. This study applies a sophisticated multi-phase flow model to simulate sedimentary density flows caused by submarine granular landslides over a simplified topography with an emphasis on their dilution mechanisms. This model takes account of the particle-particle interactions, the particle-fluid interactions, and the turbulent motions of fluid and particles in the momentum transports. The two effects on turbulence modulation are considered: one is due to the stratification and the other is caused by the correlation between the fluid and sediment turbulent motions. The numerical results suggest that the dilution mechanisms are highly related to Kelvin–Helmholtz vortices. In addition to erosion of the hyperconcentrated region by vortices, this study finds that strong stir in the concentrated region by the vortices is another important dilution mechanism. Compared with the outer regions of the vortices (with zero vorticity), the vortex cores (with non-zero vorticity) have less efficient dilution processes due to rigid-body rotations there. The vortex cores can trap the small particles for a while. The involved flow regimes are also presented. The numerical results reveal that the turbidity currents can be either turbulent or laminar when the Bagnold limit of concentration is adopted to define the boundary of turbidity currents.

Characteristics analysis of granular landslide using shaking table model test

A small-scale shaking table model test was used to investigate the characteristics of the granular landslide deposits under influences of seismic wave. The model was subjected to horizontal and vertical seismic waves. The maximum displacement, width, thickness and area are measured. The results show that vibration frequency significantly influences the deposit shape. The maximum displacement, width, thickness and area of the deposit increase with the enhanced frequency when the vibration orientation is horizontal. The maximum displacement of granule is relatively smaller when the vibration orientation is vertical, while the deposit width and thickness turn out to be greater. Other characteristics of landslide deposits under different seismic waves and some preliminary guidelines for design of engineering protective structures are also provided in this study.

Waves and Sediment Transport Due to Granular Landslides Impacting Reservoirs

AbstractGranular landslides impacting reservoirs may generate large waves and cause active sediment transport, and an increased understanding of these processes is important for public safety and effective reservoir management. This study investigates the waves and sediment transport caused by landslides impacting reservoirs using a two‐dimensional coupled double‐layer‐averaged shallow water hydro‐sediment‐morphodynamic model. In contrast to existing models, which cannot fully account for sediment transport, the model makes a physical step forward. The model is benchmarked against laboratory experiments of landslide‐generated waves in both two and three dimensions. Based on extended numerical cases, the capability of the model is further demonstrated by comparisons with empirical relationships of waves in 2‐D. In addition, sediment transport is resolved in terms of the sediment concentration and bed deformation. The results show that the wave types and amplitudes in 2‐D are dictated by the sediment transport speed, which also governs the landslide‐to‐wave momentum transfer and the landslide efficiency, which is defined as the ratio of the horizontal runout distance to the vertical fall height. With increasing sediment transport speed, landslide‐generated waves in 2‐D vary gradually from smaller nonlinear oscillatory waves to larger waves with solitary‐like wave characteristics, including nonlinear transition waves, solitary waves, and dissipative transient bores. In contrast to the momentum transfer ratio, the landslide efficiency increases with the sediment transport speed and decreases with the reservoir water depth and the lateral spreading in 3‐D cases.

Impulse waves generated by subaerial landslides of combined block mass and granular material

Experimental results, based on two-dimensional (2-D) laboratory tests, for impulse waves generated by subaerial landslides of combined solid block and granular materials are presented in this study. The results are compared with those of individual models of pure solid block and granular landslides. By considering an identical slide mass and release height, the effect of the mass ratio m* on wave generation was investigated. The mass ratio is defined as m* = MB/M, where MB is the mass of the solid block, M = MB + MG is the total mass and MG is the mass of the granular portion. The experimental results show that the combined landslides with m* = 0.6 and 0.8 in this study generally produce much larger impulse waves in the impact zone compared with those triggered by pure solid block landslides (m* = 1) and pure granular landslides (m* = 0). This suggests that the primary wave amplitudes of impulse waves might have been underestimated in previous laboratory tests with solely solid or granular assemblies when using the same slide mass and release height. The larger primary wave amplitudes of the combined landslides compared with those of the pure solid block landslides are mainly attributed to the relatively large thickness of the combined landslides and the continuous motion of the granular portion, as the inside solid block stops at the hill slope toe. Compared with pure granular landslides, combined landslides generally have a larger Froude number and slide thickness, which account for the larger primary wave amplitudes. The latter plays a quantifiably more important role – nearly two times that of the former. The effects of the hill slope angle α, and the grain size of the granular materials D, were also studied in this work. Four different hill slope angles (α = 22.5°, 30°, 37.5° and 45°) and four different mean grain diameters (D = 5 mm, 10 mm, 20 mm and 30 mm) were tested. By comparing the measured maxima of the wavelet spectra obtained via a wavelet transform method with those reconstructed by means of the phase celerity of the solitary wave cs and the group celerity of linear wave cg(f), it is found that the measured maxima travel at either cs or cg(f), depending on the landslide type and the hill slope angle α. However, the celerity of the measured maxima for the high frequency is lower than cg(f) and cs for the impulse waves generated by combined landslides when α ≤ 37.5° in this study. Further investigations show that the wave amplitude decreases rapidly during wave propagation, following an exponential function. The decrease in the wave amplitude during propagation is mainly attributed to dispersion.

A general numerical model for surface waves generated by granular material intruding into a water body

Surface water waves generated by large-scale landslides have been a major concern of many geoscientists and coastal engineers because they may result in disastrous consequences. This study presents an advanced two-phase model for dry granular material intruding into an otherwise still water body as well as their resulting waves. The water-air interface both within and outside the granular material is captured by the volume of fluid method. The inter-granular stresses are formulated based on a general collisional-frictional law developed for underwater granular flows and a modified k−ε model is adopted to describe the turbulence effect of the ambient fluid. Phase interaction is characterized by the drag force caused by the relative motion between the granular particles and the fluid. The effect of the ambient fluid on the restitution coefficient of granular particles is also considered. The newly proposed two-phase model is validated following a reasonable agreement of the numerical results with measured data from small-scale laboratory tests on surface waves caused by collapse of a subaerial granular column and by intrusion of a landslide into water. Generation and propagation of the waves, as well as the motion and deformation of the granular body, are all adequately represented by the numerical model. A relatively more general applicability of the proposed model to the study of waves generated by granular landslides can thus be expected.

Waves and sediment transport due to granular landslides impacting reservoirs

Waves and sediment transport due to granular landslides impacting reservoirs

Waves and sediment transport due to granular landslides impacting reservoirs

Waves and sediment transport due to granular landslides impacting reservoirs

Tsunamis generated by fast granular landslides: 3D experiments and empirical predictors By Francesco Bregoli, Allen Bateman and Vicente Medina

The Authors conducted experiments on tsunami generation by fast granular landslides in a 3D wave basin. They developed a novel measuring system based on computer vision, allowing for adaptively tra...

Laboratory experiments on three-dimensional deformable granular landslides on planar and conical slopes

Landslides of subaerial and submarine origin may generate tsunamis with locally extreme amplitudes and runup. While the landslides themselves are dangerous, the hazards are compounded by the generation of tsunamis along coastlines, in enclosed water bodies, and off continental shelves and islands. Tsunamis generated by three-dimensional deformable granular landslides were studied on planar and conical hill slopes in the three-dimensional NEES tsunami wave basin at Oregon State University based on the generalized Froude similarity. A unique pneumatic landslide tsunami generator (LTG) was deployed to control the kinematics and acceleration of the naturally rounded river gravel and cobble landslides to simulate broad ranges of landslide shapes and velocities along the slope. Lateral and overhead cameras are used to measure the landslide shapes and kinematics, while acoustic transducers provide the shape of the subaqueous deposits. The subaerial landslide shape is extracted from the camera images as the landslide propagates under gravity down the hill slope, and surface reconstruction of the landslide is conducted using the stereo particle image velocimetry (PIV) system on the conical hill slope. Subaerial landslide surface velocities are measured with a planar PIV system on the planar hill slope and stereo PIV system on the conical hill slope. The submarine deposits are characterized by the runout distances and the deposit thickness distributions. Larger cobbles are observed producing hummock type features near the maximum runout length. These unique laboratory landslide experiments serve to validate deformable landslide models as well as provide the source characteristics for tsunami generation.

Runup of granular landslide‐generated tsunamis on planar coasts and conical islands

AbstractLarge‐scale physical model experiments of tsunamis generated by granular landslides and volcanic flank collapses are conducted to study the wave runup on both the hill slope laterally adjacent to the landslide and an opposing hill slope. A pneumatic landslide tsunami generator was deployed on planar and convex conical hill slopes to simulate deformable landslides with various geometries and kinematics. On the landslide hill slope, maximum runup and rundown were observed in the landslide impact region followed by adjacent second maxima after the lateral waves were fully formed. The runup and rundown decayed asymptotically from the second maxima. In the conical island scenario, a localized runup amplification was measured on the lee side of the island. Outside the landslide impact region, the effects of the landslide granulometry on the lateral wave runup are minimal. The lateral wave runup on the planar hill slope was generally larger than on the convex conical hill slope outside the landslide impact region. The convex conical hill slope traps less lateral wave energy. The zeroth mode of the edge wave dispersion relation matched the first and second lateral waves on the planar hill slope and the first wave on the convex conical hill slope. Predictive equations for the laterally propagating wave characteristics are derived and a method to predict the runup on an opposing hill slope is presented. The predictive wave and runup equations are benchmarked against the 2007 landslide‐generated tsunami in Chehalis Lake, British Columbia, Canada.

On the transfer of momentum from a granular landslide to a water wave

The momentum flux from a landslide at impact in a water body is the driving force behind the generation of landslide tsunamis. Analysis of this problem is presented and used to derive idealized theoretical relationships for the maximum wave amplitude in the near-field zone. This is accomplished for momentum transfer using both hydrostatic and hydrodynamic assumptions. For rapidly evolving near-field waves with a supercritical densimetric Froude number, the maximum wave amplitude is also constrained according to the fluid continuity equation. Simplification of the hydrostatic momentum equation is also presented by considering fluid acceleration, resulting in an expression that is independent of length and time scales during wave generation. The results of the novel momentum-based equations are in agreement with laboratory data collected using high-speed digital cameras for granular landslides and previously published experimental data used to develop a semi-empirical equation. Furthermore, the results provide new insight on the range of applicability of theoretical and semi-empirical equations for predicting the maximum near-field wave amplitude of landslide-generated tsunamis.

Tsunamis generated by fast granular landslides: 3D experiments and empirical predictors

ABSTRACTLandslides falling into water bodies can generate impulsive waves, which are a type of tsunamis. The propagating wave may be highly destructive for hydraulic structures, civil infrastructure and people living along the shorelines. A facility to study this phenomenon was set up in the laboratory of the Technical University of Catalonia. The set-up consists of a new device releasing granular material at high velocity into a wave basin. A system employing laser sheets, high-speed and high-definition cameras was designed to accurately measure the high velocity and geometry of the sliding mass as well as the produced water displacement in time and space. The analysis of experimental data helped to develop empirical relationships linking the landslide parameters with the produced wave amplitude, propagation features and energy, which are useful tools for the hazard assessment. The empirical relationships were successfully tested in the case of the 2007 event that occurred in Chehalis Lake (Canada).

Tsunamis generated by long and thin granular landslides in a large flume

AbstractIn this experimental study, granular material is released down slope to investigate landslide‐generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near‐field wave, the far‐field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05 to 0.50 m. Observations from high‐speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the “effective mass”) is engaged in activating the leading wave. The wave behavior is highly dependent on the water depth relative to the size of the landslide. In deeper water, the near‐field wave behaves as a stable solitary‐like wave, while in shallower water, the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near‐field wave amplitude, the far‐field amplitude, and the runup of tsunamis generated by granular landslides.